Generally, when optical fibers are to be connected to each other by fusion splicing, a covering material is removed from an end of each optical fiber by a predetermined length to expose a cladding. In such a state, those two optical fibers are, connected to each other by fusion splicing. (See, e.g., Patent Literature 1.) FIG. 1 shows two optical fibers fused in such a manner. With the structure shown in FIG. 1, two optical fibers 510 and 610 are fused together within a space surrounded by a reinforcement member 500. A downstream end of a covering material 520 of the optical fiber 510 is removed over its whole circumference, and an upstream end of a covering material 620 of the optical fiber 610 is removed over its whole circumference. An exposed end of a cladding 530 of the optical fiber 510 and an exposed end of a cladding 630 of the optical fiber 610 are fused at a fusion splicing point 700.
At the fusion splicing point 700 between the optical fibers 510 and 610, light propagating through the core of the optical fiber 510 may leak out to the cladding 630 of the output optical fiber 610 due to microbend or axial misalignment caused at the fusion splicing point 700. When general optical fibers are fused together, significant problems do not occur because the power of light propagating through the core is not so high. In optics through which high-power light propagates, such as a fiber laser, however, high-power leakage light is produced in the cladding 630 of the output optical fiber 610 even if only slight misalignment is caused to the fusion splicing point 700.
If the covering material 620 of the output optical fiber 610 has a refractive index higher than a refractive index, of the cladding 630, the leakage light produced in the cladding 630 of the optical fiber 610 is introduced into the covering material 620 from the cladding 630 and absorbed therein.
With the structure shown in FIG. 1, however, the covering material 620 covers the whole circumference of the cladding 630. Therefore, the leakage light produced in the cladding 630 is locally emitted to the covering material 620 at the most upstream part 642 of the covering material 620 that covers the cladding 630. Accordingly, the leakage light is locally absorbed in the covering material 620 at the most upstream part 642. This absorption may cause generation of heat or a serious accident such as fire.